1. Field of the Invention
This invention relates to a semiconductor device including a power MOS field-effect transistor (herein-after referred to as “power MOSFET”) and a driver circuit driving the power MOSFET. In particular, this invention relates to a non-insulated DC-DC converter for high-speed switching, including a power MOSFET and a driver circuit driving the power MOSFET.
2. Description of the Related Art
With reduction in voltages of power sources used for CPUs (central processing unit) of computers and the like, synchronous rectifying power sources have been in heavy use. Further, the current change rate (di/dt) required of power sources for CPUs is further increasing. Also to suppress ripples in output voltages of power sources, it is important to enhance the speed of power sources.
FIG. 1 shows a circuit diagram of a conventional DC-DC converter which transforms a direct-current voltage. In the conventional DC-DC converter, a high side MOS field-effect transistor (High Side FET) 101 formed of a discrete element, a low side MOS field-effect transistor (Low Side FET) 102 also formed of a discrete element, and a driver circuit 103 which drives them are enclosed in separate packages, and connected to one another on a printed circuit board (for example, please refer to MAX1710 evaluations kit, “Products Catalogue: Maxim Integrated Products”, Maxim Japan K.K., 1998, p. 1-7).
However, as the current change rate (di/dt) increases, lowering of the conversion efficiency (output electric power/input electric power) due to parasitic inductance 104 on the printed circuit board and parasitic inductances 104 in packages caused by bonding wire are so large that it cannot be ignored.
Further, with increase in the speed, an output resistance of gate resistance and driver resistance existing in discrete elements causes decrease in the conversion efficiency of the DC-DC converter. The low side FET is turned on/off when the drain-source voltage is “0”, and thus no switching loss occurs. In the meantime, the high side FET is turned on/off with change in the drain-source voltage, and thus switching loss occurs. Therefore, decrease in the conversion efficiency due to increase in the parasitic inductance and resistances is larger in the high side FET.